The present invention relates to an optical reading apparatus for detecting recorded information signals from optical disks.
For detecting information from an optical disk, a laser beam is focused on the disk and light reflected from it is detected by a photodetector, the ouptut of which is a logic-1 or logic-0 representing respectively the presence or absence of a microscopic pit on the disk. To obtain readout signals as well as tracking information, the surface of the photodetector is equally divided into four segments. The outputs of the segments aligned in a direction perpendicular to the track are applied to a tracking control ciruit and the outputs of all the segments are combined in a single summing amplifier to produce a readout signal. However, due to the finite size of the beam spot, the amount of reflection from the disk varies significantly as the beam falls on an edge of a pit and, as a result, the waveform of the readout signal has a large rise time at the leading edge and a large fall time at the trailing edge. The lack of sharpness at the edges of the photodetector waveform is likely to result in undesirable interference between adjacent information symbols if the storage density of pits is increased.